A super absorbent polymer (SAP) is a type of synthetic polymeric materials capable of absorbing moisture in 500 to 1000 times its own weight. Various manufacturers have been denominated it as different names such as “Super Absorbency Material” (SAM) or “Absorbent Gel Material” (AGM). Since such super absorbent polymers started to be practically applied in sanitary products, they have been widely used for hygiene products such as disposable diapers for children, water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, and materials for poultice and the like.
The super absorbent polymers can be typically prepared in a powdery form by polymerizing the monomers for resins, and then drying and grinding the same.
In the process for preparing the super absorbent polymers, a step of polymerizing the monomers is critical for determining the properties of polymers. For such polymerization methods, a reverse phase suspension polymerization, a thermal polymerization, and a photo-polymerization have been known in the art. Among them, some photo-polymerization method involves polymerizing a monomer composition by placing the monomer composition for polymers on a belt and irradiating the same with a light from the top.
However, during such polymerization as described above, the amount of irradiation can vary with a depth in the monomer composition, resulting in an uneven degree of polymerization depending on the location. For example, when a predetermined thickness of the monomer composition is placed on a belt, the top of the monomer composition located closely to a light source obtains much of light irradiation, thereby undergoing polymerization relatively in excess. By contrast, the bottom of the monomer composition located away from the light source fails to secure a sufficient amount of light irradiation so that the polymerization cannot proceed completely and leaves behind the monomers as not be polymerized. In such case, the properties of the super absorbent polymers can deteriorated. In a bid to address such problems, one can change the light intensity or the light irradiation time depending on the position in a monomer composition solution, but this measure can make the process more complicated and lead to a longer processing time, lowering the productivity.